High brightness fillers are used extensively in the manufacture of various products such as specialty papers where special properties cannot be achieved with the usual clay fillers. Some of these special properties include high brightness, high opacity at low basis weight, high fidelity printing, etc. The numerous, and now relatively frequent increases in postal rates have stimulated increased use of high brightness fillers having high opacity and low print show-through at low basis weight in paper for use in printing commonly mailed items such as magazines.
Prior to the present invention, high brightness fillers were produced in a variety of pigment types from a number of different natural minerals as well as synthetic materials. Also, a few highly refined and specially processed clays are sometimes used as a high brightness filler, e.g., calcined kaolin.
One high brightness filler is made by fusing sand and soda ash to form sodium silicate. Afterward the sodium silicate is dissolved in water and calcium hydroxide is added to form amorphous calcium silicate which precipitates in the form of finely divided particles. Although this material is a satisfactory high brightness filler, it is expensive to produce, particularly because of the energy required to fuse the starting materials and to dry the final product.
Titanium dioxide, in either anathase or rutile crystal form, is an excellent opacifier, however, its high cost and relatively short supply limit its use in many areas.
It is known to produce a high brightness filler by grinding talc containing tremolite in mechanical mills such as a Raymond ring-roller mill followed by a Raymond vertical mill and to remove the larger particles of the resultant product with a cyclone or centrifugal classifier. Although such products have many uses their brightness and their ability to opacify are not as high as desired for many applications.
It is also known to mill talc mineral to very fine particle sizes using either high pressure steam in what are commonly known as jet mills or in vibratory mills using spherical grinding media. These processes are disclosed in U.S. Pat. Nos. 3,366,501, 3,643,875 and 3,476,576. The process disclosed in U.S. Pat. No. 3,366,501 is undesirable because it requires a calcining step which is costly, particularly in view of the present fuel shortage. Similar particle size distributions to those produced by the two processes disclosed in U.S. Pat. Nos. 3,476,576 and 3,643,875 do not opacify as well as might have been expected from a mathematical analysis of their particle size distributions using the Mie theory. One reason may be that this theory assumes the particles are all spherical and of equal size, which of course is not true.